The present embodiments relate generally to an improved photochromic medium that can be used for reimageable documents. More particularly, the present embodiments are directed to a photochromic material for transient documents which have an “erase-on-demand” capability.
Many paper documents are promptly discarded after being read. Although paper is generally perceived as inexpensive, the quantity of discarded paper documents is enormous and the disposal and recycling of these discarded paper documents raises significant cost and environmental issues. The present invention addresses the above described problem by providing in embodiments an improved photochromic medium for containing the desired image, a new method to prepare such a medium, and a new image forming method.
Imaging techniques employing photochromic materials, that is materials which undergo reversible or irreversible photoinduced color changes are known, for example, U.S. Pat. No. 3,961,948 discloses an imaging method based upon visible light induced changes in a photochromic imaging layer containing a dispersion of at least one photochromic material in an organic film forming binder and is hereby incorporated by reference. These and other photochromic (or reimageable or electric) papers are desirable because they can provide imaging media that can be re-used many times, to transiently store images and documents. For example, applications for photochromic based media include reimageable documents such as, for example, electronic paper documents.
Transient documents and photochromic paper for transient documents have already been reported in U.S. Pat. Nos. 7,300,727 and 7,214,456, where images printed on photochromic paper can be read for a few hours under room light conditions, and the transient documents self-erase with no effort from the user and are ready to be printed again with new images the next day. The resulting blank sheet of paper is ready to be printed again with new information, and the paper may be reusable many times.
A different application for photochromic based media is for reimageable documents, and as mentioned above, for electronic paper documents. Reimageable documents require information to be kept for as long as the user wants, then the information can be erased or the reimageable document can be re-imaged using an imaging system with different information. Written images on electronic paper may be readable for longer period of time such as, for example, many days or weeks. For example, in U.S. Pat. No. 7,316,875, incorporated herein by reference, it was discovered that solutions in acetone of spiropyrans modified with chelating groups in the presence of metallic cations had life-times of at least several days. In contrast, a classical spiropyran molecule, containing no chelating groups, is stable in its colored form for only a few minutes.
However, photochromic materials based on spiroyrans are fast “self-erasing” in that once an image is created, the colorant decays by both thermal and photochemical processes to the colorless state. Longer life-time may be desirable in that an image formed on a medium remains stable for a longer period of time. For example, a user may want to view an image for a longer period than a few hours. This is the case for electronic paper documents, which should maintain a written image for as long as the user needs to view it. The image may then be erased or changed with a different one by the user on command. Thus, there is a continued need for devising a photochromic medium that can be used for reimageable documents with an “erase-on-demand” capability.